Various means have been used in the prior art to propel a 30 mm. grenade type projectile from a gun. The prior art 30 mm "cased" system uses a typical high-low cartridge case. In the prior art device, a primer is struck which ignites a propellant contained in a small confined volume. This small volume allows the build-up of high pressure (7-10,000 psi.) to insure proper propellant burning. Since large-base-area heavy grenades require relatively low pressure for proper launch, the confined high-pressure gas generated by a propellant is released behind the grenade into a low pressure chamber through a series of orifices from a high pressure chamber. The problem with these prior art grenade launchers, which utilize a cartridge case having both high and low pressure chambers, is that they are inefficient for use in a hand carried weapon because of their excessive weight and size.